What happens if a sitter cancels during a house sitting booking?

I believe, if the sitter got injured and needed an emergency surgery etc while sitting for you, she would inform you and Rover, and Rover would find a substitute. You should call Rover directly and ask though.

I'd highly recommend choosing an alternate sitter and confirming that person would be willing to care for your pets while away if needed, and communicate that to the primary sitter you're trusting to care for your pets. This can even be documented in rover messages, so support can reference them in order to facilitate if needed. That way, you can have peace of mind while you're traveling that if the need should arise, the transition process flows smoothly and you'll be more confident about the pets' care.